In recent years, in the diagnosis of infectious diseases, diagnostic kits for detecting pathogens such as bacteria and viruses are in widespread use. Among these diagnostic kits, a diagnostic kit utilizing an immunochromatography method is used widely because it can achieve measurement conveniently and rapidly. The principle of the measurement according to the immunochromatography method carried out using the diagnostic kit is as follows. First, a sample analysis tool (test piece) formed of a porous membrane and in which antibodies are immobilized on a measurement region of the porous membrane is provided. A sample (specimen) and labeled antibodies labeled with colored particles are added thereto. When antigens as a substance to be detected are present in the sample, the labeled antibodies and the immobilized antibodies form complexes via the antigens, whereby the measurement region on the porous membrane is colored by the colored particles, for example. The label may be, in addition to the colored particles, an enzyme, which is used in combination with a substrate that is colored through an enzyme reaction. The measurement region generally is in a line form. When coloring is observed, it is regarded that the substance to be detected is presence in the sample so that it is determined that the sample is positive. On the other hand, when no coloring is observed, it is regarded that the substance to be detected is not presence in the sample so that it is determined that the sample is negative. Such determination can be made through visual observation. However, in terms of objectivity and higher efficiency in determination, etc., an immunoassay analyzer that carries out determination by optically detecting the coloring of a measurement region of a sample analysis tool has been put to practical use (e.g., Patent Document 1 and the like). However, in the diagnostic kit utilizing the immunochromatography method, abnormal coloring, e.g., coloring due to a cause other than immunoreactions, such as coloring due to a nonspecific reaction, discoloration due to degradation of a reagent with time, adhesion of dust or the like, may occur. According to visual observation, it is possible to discriminate such abnormal coloring from normal coloring. However, the immunoassay analyzer erroneously may detect such abnormal coloring.